The phrase “strained silicon complementary metal oxide semiconductor (CMOS)” essentially refers to CMOS devices fabricated on substrates having a thin strained silicon (strained Si) layer on a relaxed silicon-germanium (SiGe) alloy layer. Electron and hole mobility in strained Si layers have been shown to be significantly higher than in bulk silicon layers, and MOSFETs with strained Si channels have been experimentally demonstrated to exhibit enhanced device performance compared to devices fabricated in conventional (unstrained) silicon substrates. Potential performance improvements include increased device drive current and transconductance, as well as the added ability to scale the operation voltage, without sacrificing circuit speed, in order to reduce power consumption.
Strained Si layers are the result of biaxial tensile stress induced in silicon grown on a substrate formed of a material whose lattice constant is greater than that of silicon. The lattice constant of germanium is about 4.2 percent greater than that of silicon, and the lattice constant of a SiGe alloy is linear with respect to its germanium concentration. As a result, the lattice constant of a SiGe alloy containing fifty atomic percent germanium is about 1.02 times greater than the lattice constant of silicon.
Epitaxial growth of Si on such a SiGe substrate will yield a Si layer under tensile strain, with the underlying SiGe substrate being essentially unstrained, or “relaxed.” A structure and process that realize the advantages of a strained Si channel structure for MOSFET applications are taught in commonly-assigned U.S. Pat. No. 6,059,895 to Chu, et al., which discloses a technique for forming a CMOS device having a strained Si channel on a SiGe layer, all on an insulating substrate.
A difficulty in fully realizing the full advantages of strained Si CMOS technology is the presence of the relaxed SiGe layer under the strained Si layer. As indicated above, the strain in the Si channel depends on the lattice constant of the SiGe alloy layer. Thus, to increase strain and mobility, SiGe with an increased Ge content is needed. The use of a high Ge content (on the order of about 35 atomic % or greater) is however problematic in CMOS device fabrication in terms of chemistry. In particular, a SiGe layer having a high Ge content can interact with various processing steps, such as thermal oxidation, doping diffusion, salicide formation and annealing, such that it is difficult to maintain material integrity during CMOS fabrication, and may ultimately limit the device performance enhancements and device yield that can be achieved.
Co-assigned U.S. Pat. No. 6,603,156 to Rim discloses a method of forming a strained Si layer directly atop an insulator layer of a silicon-on-insulator substrate. The method disclosed in the '156 patent overcomes the drawbacks in the prior art by completely removing the SiGe alloy layer from the structure. Although the '156 patent provides an alternative to the problems of strained Si/relaxed SiGe heterostructures, there is still a need to provide a method that decouples the preference for high strain in the strained Si layer and the Ge content in the underlying SiGe alloy layer. Such a method would allow for continued use of strained Si/SiGe heterostructure technology.